This invention relates to measurement of fluid flow characteristics and, more particularly, to a method and apparatus for measuring the density of flowing fluid and, optionally, if desired, the mass flow rate of the fluid.
In the process control field, there is a need to measure density of a fluid. A device for accomplishing this result is called a densitometer or a densimeter. Generally, fluid density is determined in one of two ways--inferentially or directly. To determine density inferentially, the fluid mass and its volume are measured and density is computed from these measurements. To determine density directly, a parameter related to density by physical law, such as buoyancy, momentum, radiation attenuation, or sonic velocity is measured. A densitometer based on buoyancy typically employs a float and an electrical transducer for sensing the float's displacement. A densitometer based on momentum typically employs a bendable arm disposed transverse to the flow stream and a strain gage to sense the arm's deflection.
One important application for a densitometer is to measure mass flow rate. For example, for petroleum custody transfer and mixing of ingredients in a chemical or physical process, mass flow rate is usually the determinative parameter. A turbine flow meter, as well as a number of other types of flow meters, measure volumetric flow rate. In a turbine flow meter, the angular velocity of the turbine closely tracks the velocity of the fluid driving the turbine blades and is thus representative of the volumetric flow rate. Because the turbine is journaled for rotation on low friction bearings, it absorbs very little of the energy of the flowing fluid and thus therefore responds rapidly to changes in volumetric flow rate. A mass flow rate of a fluid is equal to its volumetric flow rate times its density. Fluid density, in turn, is dependent upon temperature and pressure, particularly when the fluid is a gas. Accordingly, it is necessary to determine fluid density in order to provide an accurate indication of mass flow rate with a turbine flow meter.